Human blood vessels grown in mice
The blood vessels are part of the circulatory system and function to transport blood throughout the body. The most important vessels in the system are the capillaries, the microscopic vessels which enable the actual exchange of water and chemicals between the blood and the tissues, while the conduit vessels, arteries and veins, carry blood away from the heart and through the capillaries or back towards the heart, respectively.
Scientists have used human cells to grow new blood vessels in a mouse for the first time, a US journal reports.
It could eventually help patients who had suffered heart attacks, they said.
A mixture of "progenitor" cells, taken from blood and bone marrow, made cells lining the vessels, and also those surrounding the lining.
A UK expert said that the Harvard research was "promising", and could eventually help lab-grown organs to be implanted successfully.
The ability to develop swiftly a new network of tiny blood vessels - known as capillaries - would be a prize for scientists.
There are dozens of potential applications in medicine, particularly in the treatment of conditions which involve damage to a tissue's blood supply, such as that to the heart muscle following a heart attack.
However, the complex structure of these vessels has slowed progress.